Dynamic convergence system

ABSTRACT

In a color television receiver using a three-gun color CRT with three sets of internal pole pieces, a separate external dynamic convergence assembly is located adjacent each set of internal pole pieces. Two of the three convergence assemblies incorporate magnetic flux rotation means for unbalancing the magnetic flux field across the associated internal pole pieces to deflect an electron beam along an axis skewed to a radial direction. The flux rotation is produced by various combinations of a single deflection coil located on only one leg of a U-shaped core, locating a single pole shoe between one leg of the core and the CRT, and connecting pairs of deflection coils in phase opposition.

Umted States Patent 11 1 1111 3,774,132 v Miller 1451 Nov. 20, 1973DYNAMIC CONVERGENCE SYSTEM Primary ExaminerGeorge Harris [75] Inventor:Marvin E. Miller, Des Plaines, Ill. Atwmey-JameS Nemcton at [73]Assignee: Warwick Electronics, Inc., Chicago,

Ill. [57] ABSTRACT [22] Filed: June 1972 In a color television receiverusing a three-gun color 21 APPL 263,632 CRT with three sets of internalpole pieces, a separate external dynamic convergence assembly is locatedadjacent each set of internal pole pieces. Two of the [52] US. Cl......,335/210, 313/77 three convergence assemblies incorporate magnetic [51]Int. Cl. H0" 7/00 fl ti means f b l i the magnetic fl Fleld of Search aro the associated internal pole pieces to de- 313/75 77; 315/13 fleet anelectron beam along an axis skewed to a radial direction. The fluxrotation is produced by various [56] References Cted combinations of asingle deflection coil located on UNITED STATES PATENTS only one leg ofa U-shaped core, locating a single pole 3,611,221 10/1971 Ikeuchi335/212 Shoe between one leg of the core and the CRT, and 3,657,6754/1972 Anthony et a1. 335/210 Conn cting pairs of deflection coils inphase opposi- 3,441,788 4/1969 Hooghordel et al. 315/13 C tiOn.3,684,989 8/1972 Ikeuchi 335/210 24 Claims, 5 Drawing Figures 32B 34B F5 PATENTEUnnvzo 191s SHEET 10F 2 FIG-4B DYNAMIC CONVERGENCE SYSTEMBACKGROUND OF THE INVENTION This invention relates to dynamicconvergence apparatus for a cathode ray tube, and more particularly toconvergence apparatus which produces unsymmetrical magnetic fieldsacross internal convergence pole pieces.

Typical dynamic convergence assemblies cause each of three electronbeams to be deflected along three radial axes which intersect at thecenter of a tri-color CRT. While radial beam movement is desirable forthe blue electron beam, which is generally orientedto that radialmovement coincide with a vertical plane, radial beam movement of the redand green electron beam produces both vertical and horizontal motion.The resulting convergence control is interactive and hence difficult toadjust. For the red and green beams, therefore, lateral rather thanradial deflection is desirable.

Various convergence apparatuses have attempted to provide non-radialdynamic convergence. For example, in Hooghordel et al U.S. Pat. No.3,441,788, lateral convergence is accomplished by using an asymmetricalconvergence system having four external pole pieces adjacent a CRT withthree sets of internal pole pieces.

While such structure provides lateral beam movement,

the configuration of the internal pole pieces is not con- SUMMARY OF THEINVENTION In accordance with the present invention, a novel convergenceassembly generates nonsymmetrical magnetic flux fields between a pair ofsymmetrical internal convergence pole pieces. The asymmetry of themagnetic flux field can be altered to suit particular applications,thereby providing radial convergence for one color beam and lateralconvergence for the remaining color beams in a color televisionreceiver. However, other combinations and configurations are possible,as desired.

Several embodiments are disclosed forgenerating a nonsymmetricalmagnetic flux field across a pair of magnetic pole pieces. In accordancewith one embodiment, a U-shaped core has a convergence coil wound ononly one leg. The leg carrying the coil contains a magnetic shoe locatedbetween the end of the core leg' and the glass envelope of the CRT. Inother embodiments, additional coils connected in phase opposition,and/or sources of phase opposed convergence signals, provide unbalancedmagnetic flux fields.

By changing the ratio of leakage flux to flux circulating through thecore of the external convergence assemblies, the amount of skewed beammovement can be adjusted. The amount of energy required toeffectconvergence is lower, resulting in a more efficient system. Also, thenumber of components has been reduced, making the convergence systemmore economical and easier to construct.

One object of this invention is the provision of a dynamic convergenceassembly for producing an unsymmetrical magnetic flux field between apair of symmetrical pole pieces.

Another object of this invention is the provision of a dynamicconvergence assembly having a U-shaped core, and a convergence coilwound on only one leg of the core in order to produce beam deflectionskewed from a radial direction.

Other objects and advantages of the invention will be apparent from thefollowing description, and from the.

drawings. While illustrative embodiments of the invention are shown inthe drawings and will be described in detail herein, the invention issusceptible of embodiment in many different forms and it should beunderstood that the present disclosure is to be considered as anexemplification of the principles of the invention and is not intendedto limit the invention to the embodiments illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of aconventional color CRT with internal convergence pole pieces, and

illustrating an external covergence system constructed in accordancewith the present invention;

FIG. 2 is a vector diagram of various possible paths of movement of thegreen and red electron beams shown in FIG. 1;

FIG. 3 is a cross-sectional view similar to FIG. 1, showing a modifiedform of the external convergence system and the magnetic fluxfieldgenerated thereby; and

FIG. 4A shows another embodiment of a single con-' vergence assembly,while FIG. 4B illustrates the electrical schematic thereof.

GENERAL SYSTEM OPERATION FIG. 1 illustrates in cross-section the neck ofa conventional three-gun color cathode ray tube (CRT) 20 incorporated ina color television receiver of known construction. The CRT has threeidentical gun structures for generating three electron beams whichimpinge blue (B), green (G) and red (R) phosphors on the face of theCRT. For convenience, similarstructures will be identified by the samereference numeral, followed by the designation B, G or R to indicatewhether the same is associated with the blue, green or red electron beamguns, respectively.

Within a glass tube envelope 22, a magnetic shield 24 divides the neckof the CRT into three compartments associated with the B, G and Relectron guns, which are symmetrical about a radial axis. In the absenceof a convergence magnetic flux field, an electron beam 30 travelsundeflected along an axis perpendicular to the plane of FIG. I andequidistant from the spaced pole pieces 26, 27. Each internal pole piece26, 27 is of generally L-shaped cross-section, comprised of a radiallyextending section, and a transverse section in contact with the glassenvelope 22. The illustrated internal pole structure is well known andis typical of many color picture tubes.

To effect dynamic convergence of the blue electron beam, a known type ofconvergence yoke assembly is provided. A U-shaped ferrite core is formedby a pair of adjacent legs 32 and 34, each of L-shape. An air gap 36separates the adjacent core legs. A vertical convergence coil is formedby two identical coil sections 38 and 40, wound on the legs 32 and 34,respectively. The coil sections 38, 40 are series connected and may becoupled to any conventional vertical convergence circuit in thetelevision receiver.

In response to convergence current flowing in coil sections 38 and 40,equal magnetic flux fields are generated in legs 32 and 34 and asymmetrical or balanced magnetic field is produced between the internalpole pieces 26B and 273. The blue electron beam 308 is deflected in adirection perpendicular to the symmetrical magnetic flux field,illustrated by the dashed lines, which exists across the mirror symmetrypole pieces. This causes the blue electron beam 30B to move in a radialdirection as illustrated by the vector arrows.

To adjust for static convergnece, a permanent magnet 42 spans the airgap 36 and has poles abutting each leg 32, 34. The permanent magnet 42may be rotated, or otherwise suitably moved with respect to theorientation of its magnetic North and South fields, in order to alterthe direction and magnitude of the steady magnetic flux field whichcirculates through the U-shaped legs 32, 34.

In prior convergence systems, the structure for the green (G) and red(R) assemblies would be identical with the blue (B) structure. Thiswould cause all three electron beams 30 to be deflected in radialdirections. While radial deflection is desirable for the blue beam 308,due to the fact that the blue gun structure is oriented in a verticalplane, radial deflection of the green 306 and red 30R electron beamsproduce vector components in both the vertical and horizontal planes.The vertical component of movement for the red and green beams isfrequently unnecessary and is, therefore, undesirable.

In accordance with the present invention, the green dynamic convergenceassembly and the red dynamic convergence assembly are modified so as toproduce a nonsymmetrical or unbalanced magnetic flux field between thepole pieces 26, 27 associated therewith. This field skews the path ofdeflection of the associated electron beam, and produces essentiallylateral green 30G and red 30R beam movement, as illustrated in FIG. 1.Each modified dynamic convergence assembly utilizes a single verticalconvergence coil 50 wound on only one leg of the U-shaped core. Thegreen convergence coil 506 is wound about core leg 34G, while the redconvergence coil 50R is wound on the core leg 32R.

In addition, the core leg carrying the single vertical deflection coil50 terminates in a face abutting a pole shoe 52 sandwiched between thecore leg and the glass envelope 22. The opposite or free core leg has nopole shoe, thus creating an air gap 54 between the end face of the coreleg and the glass envelope 22. The air gap is substantially the samewidth as the pole shoe 52. The net effect of the single vertical coil50, the pole shoe 52, and the air gaps 36 and 54 is to change themagnetic flux pattern produced in the vicinity of the electron beam 30.The magnetic flux fields, as illustrated by the dashed lines adjacentthe electron beams 306 and 30R, are no longer symmetrical or balancedbetween the pole pieces 26 and 27.

The flux shift results because the magnetic flux passing through thecore legs 340 and 32R is considerably greater or more concentrated thanthe flux passing through the opposed core legs 32G and 34R. Althoughleakage flux was present in prior convergence assemblies, the balancedor symmetrical construction of the assemblies, such as shown by the blueassembly, produces a balanced leakage flux distribution which results ina total flux distribution between the poles pieces which is balanced andproduces radial beam movement.

In FIG. 2,-the effect of the nonsymmetrical magnetic fields isillustrated for the green 300 and red 30R electron beams. The vectors 60illustrate a radial path of beam deflection, which would be produced bya balanced coil design such as the blue convergence assembly. When onlythe green convergence coil 50G of FIG. I is energized, the greenelectron beam 300 has a path of movement as illustrated by vector 620.Similarly, when only the red convergence coil 50R of FIG. 1 isenergized, the red beam 30R has a path of movement as illustrated byvector 62R.

However, when both deflection coils 50G and 50R are energized at thesame time, the paths of deflection of electron beams-30G and 30R aresubstantially parallel and lateral, as shown by vectors 646 and 64R.This improved efficiency when both coils 506 and 50R are energizeddemonstrates the importance of the leakage I flux in determining thedirection of beam movement. It should be noted that leakage flux isdefined as flux which does not emanate from either of the faces or polesof the core legs 32, 34. The ratio between the leakage flux and the fluxflowing through the core into the pole pieces, is important indetermining the direction of the magnetic flux lines in the vicinityofthe electron beam, and hence the direction of beam movement.

MODIFIED EMBODIMENTS In FIG. 3, modified green and red convergenceassemblies are illustrated in which the single convergence coils 50G and50R are located on the upper core legs 320 and 34R, respectively. Forclarity, the blue convergence structure is not illustrated. No pole shoeis located between the core leg carrying the coil 50 and the glassenvelope 22, reducing the imbalance. The FIG. 3 embodiment is desirablewhen a particular cathode ray tube requires maximum vertical deflectionto correct for convergence errors. A substantial flux component extendsbetween core legs 32G and 34R, via the magnetic shield 24 and across theinternal pole pieces 26B and 278. This flux component is also found inthe embodiment of FIG. I, but extends between core legs 340 and 32R, andis generally external and adjacent to the glass envelope 22. In bothcases, the leakage pattern is in a direction which assists in producingthe desired beam deflection.

The skewed angle between radial beam deflection and the beam deflectionproduced by the apparatuses of FIGS. 1 and 3 can be increased and/orselectively controlled by use of the apparatuses shown in FIG. 4.Additional rotation of the deflection vector is possible by furtherunbalancing the magnetic circuit. This can be achieved by increasing thecore gap (or reluctance amplitude) by mechanical or electrical means.

In FIG. 4A, for example, the otherwise unused ferrite leg 34 has a smallcoil connected in series opposition to a main coil 82, which otherwisecorresponds to the coil 50 of FIGS. 1 and 3. The opposing serieselectrical connection between the coils 80 and 82 is illustrated in FIG.4B. The resulting flux opposition produced by the apparatus of FIG. 4results in additional rotation of the flux field.

Other modifications to the embodiments disclosed herein which effectrotation of the flux between the internal pole pieces of a cathode raytube by unbalancing the external magnetic circuit will be apparent tothose skilled in the art. Such modifications are considered to be withinthe scope of the invention.

I claim:

1. A dynamic convergence assembly for deflecting an electron beampassing through a region bounded by two pole pieces having generallyplanar surfaces defining an axis equidistant therebetween, comprising:

core means having a first leg adjacent one of said pole pieces and asecond leg adjacent the other of said pole pieces,

coil means wound about said core means for generating a magnetic fluxfield, and

unbalance means for producing a more concentrated magnetic flux field inone of said legs to cause an unsymmetrical distribution of magnetic fluxbetween said pair of pole pieces, thereby deflecting said electron beamat a skew with respect to said equidistant axis.

2. The dynamic convergence assembly of claim 1 wherein said unbalancemeans comprises means mounting said coil means about only one of saidcore legs.

3. The dynamic convergence assembly of claim 2 wherein said unbalancemeans includes a pole shoe located between the core leg having the coiland the pole piece adjacent thereto, with the distance between thesecond leg and the pole piece adjacent thereto forming an air gap.

4. The dynamic convergence assembly of claim 2 wherein said core meanscomprises a generally U- shaped core with the first and second legsbeing generally L-shaped sections symmetrically disposed about saidequidistant axis, the closest legs of the pair of L- shape sectionsbeing spaced apart to form an air gap therebetween, said single coilbeing wound about only one of said L-shaped sections.

5. The dynamic convergence assembly of claim 1 wherein said coil meanscomprises a first coilsection wound about one of said core legs and asecond coil section wound about the other of said core legs, and

"6 legs adjacent the associated pairs of pole pieces associated with thethird electron beam controlled thereby, third coil means wound aboutsaid third core means,

and balance means for causing equal magnetic flux fields in said firstand second legs of said third core means to thereby produce asymmetrical flux distribution.

8. in a color television receiver having a cathode ray tube with threeelectron beams respectively passing through regions bounded by threepairs of internal pole pieces, each pair comprising two pole piecessymmetrically disposed on opposite sides of an axis equidistanttherebetween, a dynamic convergence system, comprising:

three convergence assemblies external to said cathode ray tube and eachlocated adjacent different ones of said three pairs of internal polepieces, each assembly generating a magnetic flux field for controllingthe deflection of the electron beam passing between the associated pairof internal pole pieces,

one of said convergence assemblies including balance means forgenerating a substantially balanced magnetic flux field between each ofthe two symmetrically disposed pole pieces associated therewith todeflect the associated electron beam along an axis parallel to saidequidistant axis, and

at least one other of said convergence assemblies including unbalancemeans for generating a substantially unbalanced magnetic flux fieldbetween each of the two symmetrically disposed pole pieces associatedtherewith to deflect the associated electron beam along an axis skewedto said equidistant axis.

9. The dynamic convergence system of claim 8 wherein each convergenceassembly includes a generally U-shaped core means having a pair ofextending legs each located adjacent a different one of the internalpole pieces associated therewith, and coil means wound on said coremeans for generating the magnetic flux field which passes through thelegs and associated internal pole pieces to control the deflection ofthe associated electron beam. I i

10. The dynamic convergence system of claim 9 wherein said balance meanscomprises a pair of coil sections each wound on a different one of saidlegs to generate substantially equal concentrations of magnetic flux insaid legs, said pair of coil sections corresaid unbalance means connectssaid first and second coil sections .in series opposition to therebyunbalance the flux distribution in the pair of core legs and associatedpole pieces.

6. The dynamic'convergence assembly of claim 1 for i -a color televisionreceiver having a cathode ray tube with a plurality of electron beamswhich individually pass through a plurality of pairs of symmetrical polepieces, first and second core means for first and second of saidelectron beams and each-having first and second sponding to said coilmeans, and said unbalance means comprises means mounting said coil meanson only one leg of the associated core means to produce a greaterconcentration of flux in the one leg.

11. The dynamic convergence system of claim 10 wherein the ends of thelegs of said other convergence associated core means to produce agreater concentra-v tion of flux in the one leg.

14. The dynamic convergence system of claim 8 wherein the remaining oneof said three convergence assemblies includes unbalance means forgenerating a substantially unbalanced magnetic flux field between eachof the two symmetrically disposed pole pieces associated therewith todeflect the associated electron beam along an axis skewed to an axisequidistant from the two symmetrically disposed pole pieces.

15. The dynamic convergence system of claim 14 wherein the unbalancemeans of said other convergence assembly and said remaining convergenceassembly cause the pair of skewed axes to be substantially parallel.

16. The dynamic convergence system of claim 14 wherein said unbalancemeans of said other convergence assembly and said remaining convergenceassembly are located to pass a stray magnetic flux field therebetween.

17. A dynamic convergence assembly for deflecting an electron beampassing between two pole pieces, comprising:

a horseshoe-type core means including a pair of side legs each adjacenta different one of said two pole pieces and an intermediate connectingleg extending between said pair of side legs at an end thereof oppositethe two pole pieces, and

a single convergence coil mounted around one of the pair of side legs.

18. The dynamic convergence assembly of claim 17 wherein saidintermediate connecting leg is interrupted by an air gap.

19. The dynamic convergence assembly of claim 17 including a secondelectron beam passing between a second pair of pole pieces, a secondhorseshoe-type core means having a pair of side legs each adjacent adifferent one of said second pair of pole pieces and an intermediateconnecting leg extending between said pair of side legs at an endthereof opposite the second pair of pole pieces, and a singleconvergence coil mounted around one of the pair of side legs of saidsecond horseshoe-type core means.

20. The dynamic convergence assembly of claim 19 wherein the singleconvergence coils are mounted on side legs which are located in facingrelationship.

21. The dynamic convergence assembly of claim 17 including a sole shoelocated between said one side leg which mounts the single convergencecoil and the pole piece adjacent thereto, an air gap between theopposite side leg and the pole piece adjacent thereto.

22. A convergence unit construction,

comprising a horseshoe-type core having a single magnetic circuitincluding a pair of legs and an intermediate connecting portionextending between said legs at one end thereof,

a single convergence coil mounted around said core.

23. A construction according to claim 22 in which said portion of saidcore extending between said legs is interrupted by a gap.

24. A construction according to claim 22 including three such coresspaced around a circle at substantially equal angular intervals,

at least two of said cores having a single such convergence coil.

1. A dynamic convergence assembly for deflecting an electron beampassing through a region bounded by two pole pieces having generallyplanar surfaces defining an axis equidistant therebetween, comprising:core means having a first leg adjacent one of said pole pieces and asecond leg adjacent the other of said pole pieces, coil means woundabout said core means for generating a magnetic flux field, andunbalance means for producing a more concentrated magnetic flux field inone of said legs to cause an unsymmetrical distribution of magnetic fluxbetween said pair of pole pieces, thereby deflecting said electron beamat a skew with respect to said equidistant axis.
 2. The dynamicconvergence assembly of claim 1 wherein said unbalance means comprisesmeans mounting said coil means about only one of said core legs.
 3. Thedynamic convergence assembly of claim 2 wherein said unbalance meansincludes a pole shoe located between the core leg having the coil andthe pole piece adjacent thereto, with the Distance between the secondleg and the pole piece adjacent thereto forming an air gap.
 4. Thedynamic convergence assembly of claim 2 wherein said core meanscomprises a generally U-shaped core with the first and second legs beinggenerally L-shaped sections symmetrically disposed about saidequidistant axis, the closest legs of the pair of L-shape sections beingspaced apart to form an air gap therebetween, said single coil beingwound about only one of said L-shaped sections.
 5. The dynamicconvergence assembly of claim 1 wherein said coil means comprises afirst coil section wound about one of said core legs and a second coilsection wound about the other of said core legs, and said unbalancemeans connects said first and second coil sections in series oppositionto thereby unbalance the flux distribution in the pair of core legs andassociated pole pieces.
 6. The dynamic convergence assembly of claim 1for a color television receiver having a cathode ray tube with aplurality of electron beams which individually pass through a pluralityof pairs of symmetrical pole pieces, first and second core means forfirst and second of said electron beams and each having first and secondlegs adjacent the associated pairs of pole pieces associated with theelectron beam controlled thereby, the first leg of the first core meansand the second leg of the second core means being located in facingrelationship, first and second coil means wound about said first andsecond core means, respectively, and first and second unbalance meansfor producing a more concentrated magnetic flux field in said facinglegs.
 7. The dynamic convergence assembly of claim 6 including thirdcore means for a third of said electron beams, said third core meanshaving first and second legs adjacent the associated pairs of polepieces associated with the third electron beam controlled thereby, thirdcoil means wound about said third core means, and balance means forcausing equal magnetic flux fields in said first and second legs of saidthird core means to thereby produce a symmetrical flux distribution. 8.In a color television receiver having a cathode ray tube with threeelectron beams respectively passing through regions bounded by threepairs of internal pole pieces, each pair comprising two pole piecessymmetrically disposed on opposite sides of an axis equidistanttherebetween, a dynamic convergence system, comprising: threeconvergence assemblies external to said cathode ray tube and eachlocated adjacent different ones of said three pairs of internal polepieces, each assembly generating a magnetic flux field for controllingthe deflection of the electron beam passing between the associated pairof internal pole pieces, one of said convergence assemblies includingbalance means for generating a substantially balanced magnetic fluxfield between each of the two symmetrically disposed pole piecesassociated therewith to deflect the associated electron beam along anaxis parallel to said equidistant axis, and at least one other of saidconvergence assemblies including unbalance means for generating asubstantially unbalanced magnetic flux field between each of the twosymmetrically disposed pole pieces associated therewith to deflect theassociated electron beam along an axis skewed to said equidistant axis.9. The dynamic convergence system of claim 8 wherein each convergenceassembly includes a generally U-shaped core means having a pair ofextending legs each located adjacent a different one of the internalpole pieces associated therewith, and coil means wound on said coremeans for generating the magnetic flux field which passes through thelegs and associated internal pole pieces to control the deflection ofthe associated electron beam.
 10. The dynamic convergence system ofclaim 9 wherein said balance means comprises a pair of coil sectionseach wound on a different one of said legs to generate substantiallyequal concentrations of magnetic flux in saId legs, said pair of coilsections corresponding to said coil means, and said unbalance meanscomprises means mounting said coil means on only one leg of theassociated core means to produce a greater concentration of flux in theone leg.
 11. The dynamic convergence system of claim 10 wherein the endsof the legs of said other convergence assembly are on approximatelyequal distance from the pole pieces, and said unbalance means furtherincludes pole shoe means located in the space between one leg end andthe associated internal pole piece.
 12. The dynamic convergence systemof claim 11 wherein said pole shoe means abuts the leg on which saidcoil means is mounted.
 13. The dynamic convergence system of claim 10wherein the remaining one of said three convergence assemblies includesunbalance means comprising means mounting said coil means on only oneleg of the associated core means to produce a greater concentration offlux in the one leg.
 14. The dynamic convergence system of claim 8wherein the remaining one of said three convergence assemblies includesunbalance means for generating a substantially unbalanced magnetic fluxfield between each of the two symmetrically disposed pole piecesassociated therewith to deflect the associated electron beam along anaxis skewed to an axis equidistant from the two symmetrically disposedpole pieces.
 15. The dynamic convergence system of claim 14 wherein theunbalance means of said other convergence assembly and said remainingconvergence assembly cause the pair of skewed axes to be substantiallyparallel.
 16. The dynamic convergence system of claim 14 wherein saidunbalance means of said other convergence assembly and said remainingconvergence assembly are located to pass a stray magnetic flux fieldtherebetween.
 17. A dynamic convergence assembly for deflecting anelectron beam passing between two pole pieces, comprising: ahorseshoe-type core means including a pair of side legs each adjacent adifferent one of said two pole pieces and an intermediate connecting legextending between said pair of side legs at an end thereof opposite thetwo pole pieces, and a single convergence coil mounted around one of thepair of side legs.
 18. The dynamic convergence assembly of claim 17wherein said intermediate connecting leg is interrupted by an air gap.19. The dynamic convergence assembly of claim 17 including a secondelectron beam passing between a second pair of pole pieces, a secondhorseshoe-type core means having a pair of side legs each adjacent adifferent one of said second pair of pole pieces and an intermediateconnecting leg extending between said pair of side legs at an endthereof opposite the second pair of pole pieces, and a singleconvergence coil mounted around one of the pair of side legs of saidsecond horseshoe-type core means.
 20. The dynamic convergence assemblyof claim 19 wherein the single convergence coils are mounted on sidelegs which are located in facing relationship.
 21. The dynamicconvergence assembly of claim 17 including a sole shoe located betweensaid one side leg which mounts the single convergence coil and the polepiece adjacent thereto, an air gap between the opposite side leg and thepole piece adjacent thereto.
 22. A convergence unit construction,comprising a horseshoe-type core having a single magnetic circuitincluding a pair of legs and an intermediate connecting portionextending between said legs at one end thereof, a single convergencecoil mounted around said core.
 23. A construction according to claim 22in which said portion of said core extending between said legs isinterrupted by a gap.
 24. A construction according to claim 22 includingthree such cores spaced around a circle at substantially equal angularintervals, at least two of said cores having a single such convergencecoil.